The conversion of methane or natural gas to carbon monoxide (CO) and hydrogen (H.sub.2) or synthesis gas by catalytic steam reforming, autothermal catalytic reforming and non-catalytic partial oxidation, is known in the prior art.
The catalytic steam reforming of natural gas or methane to synthesis gas, or hydrogen and carbon monoxide is a well established technology practiced for commercial production of hydrogen, carbon monoxide and syngas (i.e. mixture of hydrogen and carbon monoxide). In this process, hydrocarbon feeds are converted to a mixture of H.sub.2, CO and CO.sub.2 by reacting hydrocarbons with steam over a catalyst (NiO supported on calcium aluminate, alumina, spinel type magnesium aluminum oxide or calcium aluminate titanate) at elevated temperature (850.degree.-1000.degree. C.) and pressure (10-40 atm) and at a steam/carbon mole ratio of 2-5 and gas hourly space velocity (based on wet feed) of about 5000-8000 per hour. This process involves the following reactions: EQU CH.sub.4 +H.sub.2 O.revreaction.CO+3H.sub.2
or EQU C.sub.n H.sub.m +nH.sub.2 O.revreaction.nCO+[n+(m/2)] H.sub.2
and EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 (shift reaction)
The conversion is highly endothermic and is carried out in a number of parallel tubes packed with catalyst and externally heated by flue gas of temperature of 980.degree.-1040.degree. C. (Kirk and Othmer, Encyclopedea of Chemical Technology, 3rd Edn., 1990 vol. 12 p. 951; Ullamann's Encyclopedea of Industrial Chemistry, 5th Edn., 1989, vol. A-12 p. 186). The main drawbacks of this process are as follows. It is highly endothermic and operated at high temperature. Hence, it is highly energy intensive. Further, the shift reaction occurring in the process leads to formation of CO.sub.2 and H.sub.2 from CO and water, thus increasing H.sub.2 /CO ratio. Since a lower H.sub.2 /CO ratio than that obtained by the steam reforming is required for certain applications of synthesis gas, secondary reformer using CO.sub.2 or O.sub.2 oxidants are frequently required to reduce the hydrogen content of synthesis gas produced by the steam reforming. Also, the supported nickel catalyst used in the steam reforming is poisoned by sulfur containing compounds present in very low concentrations in the feed hydrocarbons. Further, there is a carbon deposition on the catalyst during the steam reforming.
Autothermal catalytic reforming of methane or natural gas with air or oxygen to H.sub.2, CO and CO.sub.2 is also an established technology. In this process, a feed gas mixture containing hydrocarbon, steam and oxygen (or air) is passed through a burner and then the combustion gases are passed over a catalyst (nickel supported on alumina) in a fixed bed reactor at 850.degree.-1000.degree. C. and 20-40 atm. (Ullamann's Encyclopedea of Industrial Chemistry 5th Edn., 1989, vol. A-12, p. 202). This process has the following drawbacks. There are large temperature and space velocity variations during start-up and shut down which leads to abrasion and catalyst disintegration, requiring frequent refilling and removal of the catalyst. This process operates at high temperature and pressure and there is a formation of carbon (or carbon deposition) in the reactor.
Non-catalytic partial oxidation of hydrocarbons to H.sub.2, CO and CO.sub.2 is an established technology used mostly for producing hydrogen from heavy fuel oils, primarily in locations where natural gas or lighter hydrocarbons, including naphtha, were unavailable or were uneconomical as compared with fuel oil or crude oil. This process is carried out by injecting preheated hydrocarbon, oxygen and steam through a specially designed burner into a closed combustion chamber, where partial oxidation of the hydrocarbons with less than stoichiometric oxygen for complete combustion occurs at very high temperature (1350.degree.-1600.degree. C.) and pressures up to 150 atm (Kirk and Othmer, Encyclopedea of Chemical Technology 3rd Edn., 1990 vol. 12 p. 952; Ullamann's Encyclopedea of Industrial Chemistry 5th Edn., 1989, vol. 12, p. 204). The main drawbacks of this process are as follows. This process is operated at a very high temperature and very high pressure and there is extensive soot or carbon formation, particularly from heavy hydrocarbons.
Recently, Ashcroft and co-workers (Nature, vol. 344, 1990, p. 319) have reported selective oxidation of methane to synthesis gas (which is a mildly exothermic reaction) using lanthanide ruthenium oxide (Ln.sub.2 Ru.sub.2 O.sub.7 where Ln is lanthanide or rare earth element such as Pr, Sm, Eu, Gd, Tb, Dy, Tm, Yb, Lu) catalysts at 777.degree. C. and total gas hourly space velocity of 4.times.10.sup.4 h.sup.-1 with CH.sub.4 /O.sub.2 mole ratio of 2.0 and N.sub.2 /CH.sub.4 mole ratio of 2.0. The catalysts were prepared by conventional solid state reactions between Ln.sub.2 O.sub.3 and RuO.sub.2 in a sealed silica tube. Although, high methane conversions to CO and H.sub.2 have been obtained using these catalysts, the catalyst cost is exorbitantly high because of the use of extremely costly Ru in the catalyst in stoichiometric quantities (i.e. Ru/Ln mole ratio=1.0).
In view of the limitations of the prior art processes and catalysts used for the production of synthesis gas or (CO and H.sub.2) from natural gas or methane, it was found desirable, during the course of the investigation leading to the present invention, to develop an improved process, which is not energy intensive (or highly endothermic), uses cheaper catalysts and operates at lower temperatures for the conversion of methane or natural gas to CO and H.sub.2 or synthesis gas.